This invention generally relates to color picture tubes and, more particularly, a method and apparatus for fabricating tension masks for color picture tubes.
A color picture tube includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the faceplate of the tube and is made up of an array of elements of three different color emitting phosphors. An aperture mask, which may be either a domed mask or a tension mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. A mask is a thin sheet of metal, such as steel, that is contoured to somewhat parallel the inner surface of the tube faceplate. A focus mask comprises two sets of conductive lines that are perpendicular to each other and separated by an insulator. When different potentials are applied to the two sets of lines to create multiple focusing lenses in each of the mask openings, the mask is referred to as a focus mask. One type of focus mask is a tension focus mask, wherein at least one of the sets of conductive lines is under tension. Generally, in a tension focus mask, a vertical set of conductive lines or strands is under tension and a horizontal set of conductive lines or wires overlies the strands.
In assembling a strand tension mask, it is required to assemble the strands with a high degree of accuracy to achieve consistent spacing between the strands.
The present invention provides a method and apparatus for maintaining uniform spacing between the strands of a tension mask frame. The apparatus includes providing a set of barrier ridges and affixing them to a tension mask. The barrier ridge is aligned perpendicular to the strands of the tension mask and affixed to the frame near the ends of the tension mask. The barrier ridges traverse the length of the tension mask and act to keep the mask strands parallel and equidistantly spaced from each other when faced with applied stress during mask welding and subsequent thermal processing. After the barrier ridges are affixed to the frame. The tension mask is mounted to the mask frame. The barrier ridges are affixed to the frame inside the frame and directly under and in contract with the mask strands. When the tension mask is mounted to the mask frame, the barrier ridges lay inside the mask frame, and between the mask frame and the array region of the tension mask that produces visible image on the screen. The mask strands are in frictional contact with the barrier ridge. They may also be adhered to the ridge by a suitable adhesive such as one sold under the trademark KASIL(trademark) containing potassium silicate.
During mask-to-frame welding, mask strands are welded directly to the cantilever of the frame. Because of the frictional force exerted by the barrier ridge, mechanical stresses applied to strands along the weld contact points are isolated to the regions of the strands of the contact points. Therefore, the portions of the strands between the barrier ridges are, advantageously, less affected by the mechanical stresses and maintain their positions. After the tension mask is affixed to the mask frame, the entire mask frame assembly is used in manufacturing a color picture tube. As such, the assembly is processed through a series of thermal cycles. The barrier ridges expand and contract in unison with the mask frame. By attaching the tension mask strands to the barrier ridges, spatial integrity between the strands is thus maintained during the manufacturing of the strand tension masks, subsequent processing and tube operation.